Blender error: out of memory
Only 25 million faces and lots of particles in 6k resolution... why would you be running out of memory?

As pointed out in the comments you have no RAM left in your system.

Working in 3D is very easy to create situations where your scene is much greater than the resources on your computer.

You really need to reassess where you need to optimize. The trick is to think of where it is that you need great detail and where you can get away with less. The art of 3D is part precision but is mostly optimization.

For example, textures that are going to be just a few pixels on the screen can be rougher than those that are going to be featured in close-up. 
Do you need that many particles or would a surface with a bump map would give you the same?

Not every strand of grass in the distance has to be a particle, The ones you don't see don't have to exist, the statues don't have to be as detailed if they are going to be so small, etc.

When Rendering on GPU you have even less resources than when you render on CPU, so it is unlikely that you will be able to use it to render such an ambitious projecet.

You need to slice the project in different layers that your computer can handle and composite later.

Here's a link that details how to do this: https://www.blenderguru.com/articles/how-to-render-a-complex-scene-without-crashing